serial: msm_geni_serial: Add enable/disable of dma irq bits

Correct the enable/disable bits of dma interrupt registers by
using DMA_IRQ_EN_SET/DMA_IRQ_EN_CLR registers respectively.

Previously DMA_IRQ_EN register was used to set and unset the
dma interrupt register bits but, DMA_IRQ_EN is a read only
register and DMA_IRQ_EN_SET and DMA_IRQ_EN_CLR are the right
registers for setting and unsetting the dma interrupt bits.

Change-Id: I0c873742b32960673320ddc9a9e3c88922a22684
Signed-off-by: Prudhvi Yarlagadda <pyarlaga@codeaurora.org>